Quantum physics is simple, mysterious, and extremely successful in explaining physical phenomena. Quantum optics, which studies the interaction of light and matter at the quantum level, has been at the heart of the development of quantum physics since the beginning and is particularly well suited for probing the unique features of quantum physics such as superposition and entanglement. We use quantum optical approaches to study potential applications of these unique quantum phenomena (e.g. a future "quantum internet"), to probe whether they are universal, and to investigate whether they could play a role in biology, especially in neuroscience.